An enquiry into the relationship between testosterone (and its metabolites) and reproduction of the male laboratory rat will be made. The following questions will be asked and the following answers are anticipated: (1) Where is Testosterone Secreted? In addition to the testis, the adrenal secretes testosterone, which increases with castration, can be measured, probably has a diurnal rhythmicity compatible with the adrenal gland, rises in castrated male rats when exposed to an estrous female and could play a significant role in the retention of sexual behavior in a castrated male rat and the maintenance of sexual behavior in an intact rat. (2) How is Testosterone Regulated? The restoration of plasma testosterone in the hemicastrated male rat is thought to be mediated by a change in LH receptors in the remaining testis, and not by changes in plasma LH. (3) What is Testosterone's Function? It primes the circuitry necessary for neuroendocrinological responses of LH, FSH and prolactin to internal and external stimuli. It regulates spermatogenesis by its presence in the cytosol and nuclei of germinal elements in the seminiferous tubules; the disproportionate percent of endogenous nuclear testosterone which is unbound is a compelling argument for a unique mechanism by which testosterone mediates its primary function. (4) Where Does Testosterone Act? It acts in the brain, is found in high endogenous concentrations in unlikely places, even in castrated male rats. Its deficit (in the face of normal plasma levels) probably leads to loss of sexual behavior in the medial preoptic lesioned male rat. Its presence in the brain fluctuates meaningfully with sexual activity. The probable independence of these local fluctuations of plasma levels provides the regulatory role of testosterone in reproduction of the male rat.